Fault recovery system and method for a communications network

ABSTRACT

A source node of a network transmits a path setup message for requesting a traffic path to a destination node. First and second nodes, located between the source and destination nodes, establishes first and second parallel transport paths on a first fault recovery layer between them. The first node further includes a database for storing identities of the first and second paths as a single virtual link as viewed from the source node. The first node responds to the path setup message for accommodating the traffic path on a second fault recovery layer through the first transport path. Each node accommodates the traffic path through the second transport path when the first transport path fails. The first node advertises recovery type of the virtual link as equal to the recovery type of the first and second transport paths, and/or equal to most unreliable recovery type of its component links.

RELATED APPLICATION

[0001] This application is a Continuation-in-Part application ofco-pending U.S. patent application Ser. No. 10/283,241, filed Oct. 30,2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to fault recoverytechniques for a communications network such as a mesh network.

[0004] 2. Description of the Related Art

[0005] In the current communications network, a fault recovery systemsuch as Automatic Protection Switching (APS) and ring fault recoveryhave been extensively used. APS is described in Chapter 3 of “FiberNetwork Service Survivability,” T. Wu, Artech House, 1992. Ring faultrecovery is described in Chapter 4 of the same publication. APS concernsfault recovery for a link connecting adjacent nodes, and a working ringand a protection ring are provisioned in advance. When a failure occursin the working ring, communication is restored by switching traffic tothe protection ring. The ring fault recovery scheme is used in a meshnetwork where a plurality of nodes are interconnected by rings. Thenetwork is segmented into a number of rings. When a failure occurs inthe network, fault recovery action is performed independently on a perring basis. While the APS method is only capable of recovering a networkfrom link failure, the ring fault recovery scheme is capable ofrecovering from both link failure and node failure.

[0006] Attention is recently focused on the mesh fault recovery schemein which the whole network is treated as a single mesh, rather than asmultiple rings. While in the ring fault recovery scheme a number ofrings cannot share a common backup resource, the mesh fault recoveryscheme allows any combination of multiple paths in a mesh network toshare a common backup resource if they meet some criteria. Therefore, inmost cases the mesh fault recovery method requires less backup resourceas compared to the ring fault recovery scheme.

[0007] These fault recovery schemes are implemented primarily accordingto SDH (synchronous digital hierarchy) and SONET (synchronous opticalnetwork) standards. However, the recent tendency is toward integratingthe control plane of MPLS (multi-protocol label switching) technologywith SDH/SONET transport networks. Known as GMPLS (generalized MPLS),routers in the GMLS network make their forwarding decision according totimeslots, wavelengths or physical ports. Mesh fault recovery scheme canbe implemented using the GMPLS technology.

[0008] In a GMPLS network, each node uses a routing protocol foradvertising link-state information indicating the identity of itsneighbor and its available network resource to every other nodes of thenetwork. Each node has its own topology database in which the advertisedlink-state information is stored and maintained. When a path isestablished, the initiation node of the path references its topologydatabase and performs a route calculation for a possible route to thetermination node of the path. When a route is determined, the initiationnode sends a signaling message along the route so that the message isable to reach every node on the route.

[0009] A mesh fault recovery using GMPLS is described in Internetdraft-lang-ccamp-recovery-01.txt submitted in IETF by Jonathan P. Lang.In this document, fault recovery is classified into path level recoveryand span level recovery. The path-level fault recovery is performed byinitiation and termination points of a path and the span-level faultrecovery is performed between adjacent nodes of a link. Fault recoverymode is classified into a 1+1 protection mode in which the traffic issimultaneously sent to working and protection routes, a 1:1 protectionmode in which the traffic is only sent to working route, and a sharedmode such as 1:N and M:N protection modes. When a failure occurs on apath, the network performs a fault location process for locating thefailure. The initiation node of the faulty path selects an alternateroute so that data may be rerouted around the trouble spot. Thealternate route may pass through a node that shares the troubled path.

[0010] If a failure occurs on an incoming link to a node where data issplit into working and protection paths, the network first identifiesthe troubled link and then proceeds to perform a fault recoveryoperation on that faulty link in a span protection mode. If the workingpath fails, the network identifies the faulty path first and thenproceeds to perform a path protection mode, in which the terminationnode of these paths switches to the protection path so that data isrerouted around the faulty spot. Because of the differences in faultrecovery mode depending on the location of path failure, the faultlocating process is an important requirement for the prior artcommunications network.

[0011] However, the need to perform a fault locating process places aburden on a network, particularly on optical networks where “opticallytransparency” is an important consideration for designing opticalcross-connect systems. Specifically, if an optical network is requiredto identify the location of failure on an optical path, the optical pathmust be monitored at strategic points along the path and the number ofsuch monitoring points would result in an add complexity to the designof optical system with an attendant increase in cost.

[0012] Another shortcoming of the prior art is that with the span-levelrecovery and path-level recovery schemes the network cannot recover fromsuch a failure that occurs in an intermediate or transit node.

[0013] Additionally, in a large mesh network, many working andprotection paths will be provided and configured in a complex pattern.The topology database of each node would be required to furnish theinformation as to the node identities and node functions of whether theyare initiation or termination points of working and protection paths.Since the network topology tends to vary with time, the topologydatabase must be updated in response to each topology variation.However, this is a formidable task to implement.

[0014] A further shortcoming of the prior art is that since theattributes of the path that carries user's traffic vary with timedepending on which of the available routes it takes to destination, theprior art path protection method is complex from the view point ofquality management for user services.

[0015] A still further shortcoming of the prior art is that it cannotperform fault recovery with a desired level of resource granularity. Ina GMPLS network, in particular, a single optical link may carrysixty-four WDM channels each transporting sixty-four TDM channels ofmultiple packet transmission paths each. If the optical transmitter ofcertain wavelength should fail, sixty-four TDM paths will be lost. Ifthe granularity of fault recovery is equivalent to a TDM path, recoveryprocess will be repeated sixty-four times to restore all TDM paths. Iffault recovery is performed at the granularity of a packet transmissionpath, the recovery process must be repeated a greater number of times.Since the recovery process of a single path involves exchanging ofsignaling messages, the signaling traffic would be enormous. Usually thesignaling channel has a narrow bandwidth. Hence it takes a long recoverytime. This is particularly true of shared path protection. On the otherhand, if fault recovery is performed for a single TDM path failure atthe granularity of a wavelength channel, the process may be simple andfree from overwhelming signaling traffic. However, sixty-three other TDMpaths would also be switched over to backup network resource, resultingin a substantial wastage of network resource.

[0016] Additionally, in a multi-domain network, precision topology datais not exchanged between different domains. There exists a need for amechanism to implement a fault recovery system that enables each domainto collaborate in a consistent manner when the network is affected by aninter-domain path failure.

SUMMARY OF THE INVENTION

[0017] It is therefore an object of the present invention to provide acommunications network having a fault recovery mechanism that solves theprior art problems and shortcomings.

[0018] According to a first aspect of the present invention, there isprovided a communications network comprising first and second nodesinterconnected by communication links, and a traffic path establishedbetween a source node and a destination node. Each of the first andsecond nodes includes routing control means for establishing first andsecond transport paths on a first fault recovery layer between the firstand second nodes. The first node further includes a database for storingidentities of the first and second transport paths as a single virtuallink as viewed from the source node, and advertises fault recovery typeof the virtual link as being equal to fault recovery type of the firstand second transport path. The first and second nodes accommodate thetraffic path on a second fault recovery layer through the firsttransport path, and accommodate the traffic path on the second faultrecovery layer through the second transport path when the firsttransport path is not working properly.

[0019] Preferably, the first and second fault recovery layers arehierarchically structured in such a relationship that no contentionoccurs between the first and second fault recovery layers when a failureoccurs in the first and second transport paths. The bandwidth of each ofthe first and second transport paths is equal to or greater than thebandwidth of the traffic path.

[0020] Preferably, each node of the network comprises a first tablememory for storing routing data of the firsts and second transport pathsand the traffic path, a second table memory for storing switching dataderived from the routing data, a switch for terminating the first andsecond transport paths, and a controller for establishing a connectionin the switch according to the switching data so that the traffic pathis accommodated in the first transport path. The controller responds toan occurrence of a failure in the first transport path for updating therouting data, and then updates the switching data according to theupdated routing data and reestablishes a connection in the switchaccording to the updated switching data so that the traffic path isaccommodated in the second transport path. The controller derives theswitching data from the routing data by translation from a control planeof the network to a transport plane of the network. When a failure isdetected in the first transport path, the routing data is updated toreflect the path failure and the switching data is updated according tothe updated routing data. The updated switching data is used toreestablish the connection.

[0021] According to a second aspect, the present invention provides acommunications network comprising first and second nodes interconnectedby communication links, and a traffic path established between a sourcenode and a destination node. Each of the first and second nodesincluding routing control means for establishing first and secondtransport paths on a first fault recovery layer between the first andsecond nodes. The first node further includes a database for storingidentities of the first and second transport paths as a single virtuallink as viewed from the source node. The virtual link comprises a seriesof component links. The first node advertises fault recovery type of thevirtual link as being equal to most unreliable fault recovery type ofthe component links. The first and second nodes accommodate the trafficpath on a second fault recovery layer through the first transport path,and accommodating the traffic path on the second fault recovery layerthrough the second transport path when the first transport path is notworking properly.

[0022] According to a third aspect of the present invention, there isprovided a multi-domain communications network comprising a plurality ofnetwork domains interconnected by inter-domain links, each of thenetwork domains including first and second nodes interconnected byinter-node links, and a traffic path established between a source nodeand a destination node. The first node of each of the domainsestablishing first and second parallel transport paths on a first faultrecovery layer to the second node of the domain and including a databasefor storing identities of the first and second parallel transport pathsas a single virtual link as viewed from the source node, andaccommodating the traffic path on a second fault recovery layer throughthe first transport path of the domain, the virtual link comprising aseries of component links. The first node advertises fault recovery typeof the virtual link as being equal to fault recovery type of the firstand second transport paths. The first and second nodes of each of thedomains accommodates the traffic path in the second transport path ofthe domain when the first transport path of the domain is not workingproperly.

[0023] According to a fourth aspect of the present invention, there isprovided a multi-domain communications network comprising a plurality ofnetwork domains interconnected by inter-domain links, each of thenetwork domains including first and second nodes interconnected byinter-node links, and a traffic path established between a source nodeand a destination node. The first node of each of the domainsestablishes first and second parallel transport paths on a first faultrecovery layer to the second node of the domain and includes a databasefor storing identities of the first and second parallel transport pathsas a single virtual link as viewed from the source node. The virtuallink comprises a series of component links. The first node accommodatesthe traffic path on a second fault recovery layer through the firsttransport path of the domain and advertises fault recovery type of thevirtual link as being equal to most unreliable fault recovery type ofthe component links. The first and second nodes of each of the domainsaccommodates the traffic path in the second transport path of the domainwhen the first transport path of the domain is not working properly.

[0024] According to a fifth aspect of the present invention, there isprovided a fault recovery method for a communications network in whichfirst and second nodes are interconnected by communication links, themethod comprises the steps of establishing first and second paralleltransport paths on a first fault recovery layer between the first andsecond nodes, storing identities of the first and second paralleltransport paths in a database as a single virtual link as viewed from asource node, advertising fault recovery type of the virtual link asbeing equal to fault recovery type of the first and second transportpaths, establishing a traffic path between a source node and adestination node, accommodating the traffic path on a second faultrecovery layer through the first transport path, and monitoring thefirst transport path, at the second node, and accommodating the trafficpath on the second fault recovery layer through the second transportpath when a failure is detected in the first path.

[0025] According to a sixth aspect, the present invention provides afault recovery method for a communications network in which first andsecond nodes are interconnected by communication links, the methodcomprising the steps of establishing first and second parallel transportpaths on a first fault recovery layer between the first and secondnodes, storing identities of the first and second parallel transportpaths in a database as a single virtual link as viewed from a sourcenode where the virtual link comprises a series of component links,advertising fault recovery type of the virtual link as being equal tomost unreliable fault recovery type of the component links, establishinga traffic path between a source node and a destination node,accommodating the traffic path on a second fault recovery layer throughthe first transport path, and monitoring the first transport path, atthe second node, and accommodating the traffic path on the second faultrecovery layer through the second transport path when a failure isdetected in the first path.

BRIEF DESCRIPTION OF THE DRAWIGNS

[0026] The present invention will be described in detail further withreference to the following drawings, in which:

[0027]FIG. 1 is a block diagram of a communications network according toan embodiment of the present invention, in which a network managementcenter is provided for centralized control;

[0028]FIG. 2 is a block diagram of the network in which transport (labelswitched) paths and a traffic path are illustrated;

[0029]FIGS. 3A and 3B are illustrations of a path table and a switchtable respectively defined in a memory device of each network node;

[0030]FIGS. 4A, 4B and 4C are flowcharts of the operation of each nodecontroller for translating routing information contained in the pathtable into switching information which is set into the switch table;

[0031]FIGS. 5A and 5B are illustrations of examples of the path tableand switch table of node B, respectively, according to the configurationof FIG. 2;

[0032]FIGS. 6A and 6B are illustrations of examples of the path tableand switch table of node M, respectively, according to the configurationof FIG. 2;

[0033]FIGS. 7A and 7B are illustrations of examples of the path tableand switch table of node N, respectively, according to the configurationof FIG. 2;

[0034]FIGS. 8A and 8B are illustrations of examples of the path tableand switch table of node C, respectively, according to the configurationof FIG. 2;

[0035]FIG. 9 is a flowchart of the operation of each node when a failureis detected in a working path;

[0036]FIGS. 10A and 10B are illustrations of the path table and switchtable of node C, respectively, when a failure is detected in the workingpath;

[0037]FIG. 11 is a block diagram of a communications network accordingto another embodiment of the present invention, in which routing andsignaling functions are distributed to the network nodes;

[0038]FIGS. 12A and 2B are illustrations of examples of the path tableand switch table of node B, respectively, of the network of FIG. 11;

[0039]FIGS. 13A and 13B are illustrations of examples of the path tableand switch table of node M, respectively, of the network of FIG. 11;

[0040]FIGS. 14A and 14B are illustrations of examples of the path tableand switch table of node N, respectively, of the network of FIG. 11;

[0041]FIGS. 15A and 15B are illustrations of examples of the path tableand switch table of node C, respectively, of the network of FIG. 11;

[0042]FIGS. 16A, 16B and 16C are illustrations of updated switch tablesof nodes C, N and B, respectively, when a failure is detected in theworking path of the network of FIG. 11;

[0043]FIGS. 17A and 17B are block diagrams of a multi-domaincommunications network showing normal and faulty states, respectively,of the working transport path;

[0044]FIG. 18 is a block diagram of a multi-domain communicationsnetwork in which wavelength and SDH layers are formed in a hierarchicalstructure;

[0045]FIG. 19 is an illustration of one example of the path table ofnode D of FIG. 18;

[0046]FIG. 20 is a block diagram of a multi-domain communicationsnetwork for illustrating routes which are taken when a desired path isnot discovered in the network of FIG. 18; and

[0047]FIG. 21 is a block diagram of a multi-domain communicationsnetwork in which inter-domain links are specified according to requestedfault recovery mode.

DETAILED DESCRIPTION

[0048] Referring now to FIG. 1, there is shown a SDH (SynchronousDigital Hierarchy) communications network according to a firstembodiment of the present invention in which the GMPLS (GeneralizedMulti-Protocol Label Switching) protocol is employed. The network iscomprised of a plurality of SDH cross-connect nodes A, B, C, D, M and Nof similar configuration, which are connected to a network managementcenter 1 via control channels. SDH signals transmitted between the nodesare multiplexed on different wavelengths and transmitted as a WDMsignal. For simplicity, only one direction of transmission is shown.

[0049] As illustrated in FIG. 1 as a representative node, the node Bincludes a cross-connect switch 2 which may be a combination of one ormore time division switching stages and one or more space divisionswitching stages. An incoming WDM signal (two STM-4 signals, forexample) from the node A is supplied to a wavelength-divisiondemultiplexer 3, where the WDM signal is demultiplexed into STM-4signals and supplied to input ports of the switch 2. In the switch 2,the time slots of an STM-4 signal supplied to one of its input ports areinterchanged in the time division stages and switched on a per-slotbasis through the space division stages to a desired output port.Therefore, the cross-connect switch 2 is able to switch an STM-1 signalsupplied to any of its input ports on any incoming timeslot to any ofits output ports on any outgoing timeslot. Obviously, the switch 2 maybe controlled to establish a connection that directly transfers an STM-4signal from one input port to any output port.

[0050] A controller 5 is provided for controlling the switch 2 toestablish connections in accordance with switching data (timeslot IDsand port IDs) stored in a switch table 8. As will be described, theswitching data is derived from routing data (attributes of paths) storedin a path table 7.

[0051] Each port of the switch 2 is identified by a unique port ID andeach incoming and outgoing STM-1 signal can be identified by one of fourtimeslot IDs of an STM-4 signal.

[0052] The output ports of optical switch 2 are connected towavelength-division multiplexers 4, where outgoing STM-4 signals arewavelength-multiplexed onto a WDM signal comprising two STM-4 signals ofdifferent wavelength channels. The WDM signals are transmitted over theoptical links that connect the node B to the transit nodes M and N,respectively.

[0053] Controller 5 of each node is connected to the network managementcenter 1 via a line interface 6 to exchange routing information forcreating a topology database in the management center 1 and setting thepath table 7 of each node with routing data.

[0054] Network management center 1 initially sends routing messages tonodes B, M, N and C to establish transport (label switched) pathsbetween nodes B and C and then sends a routing message to node A toestablish a traffic path through one of these transport paths from nodeA to node D. It is assumed that 1+1 path protection is performed. Thebandwidth of each of the transport paths is equal to or greater than thebandwidth of the traffic path.

[0055] For the purpose of explanation, it is assumed that the network isconfigured so that the functions of the management center 1 aredistributed to all network nodes. Initially, the node B performs a firstroute calculation to find an STM-4 route to node C via transit node Mand sends a signaling message to node C via node M to establish a firsttransport path 10. Node B then performs a second route calculation tofind an alternate STM-4 route to node C via node N and sends a signalingmessage to node C via node N to establish a second transport path 11.Then, the node B advertises the first and second transport paths 10 and11 as a single “logical (virtual) link” to the network, using a routingprotocol and the fault recovery type of the virtual link is advertisedas equal to the fault recovery type of the transport paths 10, 11. Ifsuch a virtual link is composed of a series of component physical orvirtual links, the fault recovery type of the virtual link is advertisedas equal to the most unreliable recovery type of its component links.

[0056] Every other nodes of the network store this information in theirtopology database. When the ingress node A performs a calculation for aroute to the egress node D with a bandwidth smaller than that oftransport paths 10 and 11, an STM-1 traffic path 12 may be found throughthe virtual link because of its presence in the topology database. Thesecond transport path 11 may be used as a protection transport path as abackup resource for the working transport path 10.

[0057] Transport paths 10 and 11 are established on a first faultrecovery layer and the traffic path 12 is established on a second faultrecovery layer. The first and second fault recovery layers arehierarchically structured with respect to each other. Preferably, thefirst and second fault recovery layers are structured so that there isno contention between them when a failure occurs in one of the first andsecond transport paths 10 and 11.

[0058] Note that, in FIG. 1, the network management center 1 performsthe above-mentioned node functions. By treating the first and secondtransport paths 10, 11 as a single virtual link, the path protectionmethod is simplified from the view point of quality management for userservices.

[0059] As shown in FIG. 2, the first and second transport paths 10 and11 are established between the node B (initiation node) and the node C(termination node) as transport paths, with the first and second pathspassing through transit nodes M and N, respectively. Each of these firstand second paths has a traffic-carrying capacity equivalent to awavelength resource. Network management center 1 sets the first andsecond transport paths 10, 11 as a single virtual link in its topologydatabase so that the virtual link can be used as a hierarchicallydistinct from the view point of fault recovery with respect to the faultrecovery of the traffic path 12. Based on this topology data, the center1 performs a route calculation for a path from node A to node D andestablishes a traffic path 12 via the virtual link. The path recoverytype of transport paths 10 and 11 are dedicated 1+1 and the pathrecovery type of traffic path 12 is unprotected. Therefore, the trafficpath 12 is not recovered from failure. Because of the dedicated 1+1fault recovery, the node B establishes connections in its cross-connectswitch 2 so that the traffic signal on the path 12 is simultaneouslyapplied to both transport paths 10 and 11, whose downstream endsrespectively appear at ports C2 and C3 of node C. Using the first andsecond paths 10, 11 as working and protection paths, respectively, thenode C establishes a connection between the port C2 and port C6 whichleads to the node D, leaving the port C3 unconnected. Therefore, thetraffic path 12 is accommodated in the working path 10 to transfer thetraffic signal via the transit node M. In this way, the first and secondpaths 10 and 11 are hierarchically structured on a different faultrecovery layer from that of the traffic path 12.

[0060] As shown in FIG. 3A, the path table 7 has a plurality of entries20 for entering attributes of a plurality of paths. Each path tableentry, which is identified by a path ID field 21, is divided into fields22 through 33 for setting the values of a virtual link interface ID,bandwidth, upstream node ID, upstream interface ID, upstream label,downstream node ID, downstream interface ID, downstream label, pathrecovery type, activity (active or inactive), accommodated path andtunnel ID (which is only used for distributed network). As will bedescribed later, the attribute data of the path table 7 are used forsetting the switch table 8 with data necessary for the controller 5 toperform switching on the incoming SDH signals transported on a WDMchannel.

[0061] The virtual link interface (I/F) ID 22 field of each path entryis used to indicate whether the path of that entry is a transport pathor not. Since the transport paths 10 and 11 are visible as a singlevirtual (or logical) link from nodes other than nodes B and C, a commonvirtual link interface ID is given to the paths 10, 11 and no VL I/F IDis given to the traffic path 12.

[0062] Bandwidth field 23 of each path entry indicates the bandwidth ofits path. In a typical example, STM-4 (equivalent to 622 Mbps) are givento paths 10 and 11 and STM-1 (equivalent to 155 Mbps) is given to path12. The upstream node ID field 24 of each path entry indicates theidentity of an adjacent node on the upstream side of the local node, andthe downstream node ID field 27 indicates the identity of adownstream-side adjacent node of the local node. Upstream and downstreaminterface ID fields 25 and 28 of each path table entry are used toindicate the port ID (i.e., identity of a physical link) or VL interfaceID of the path on the upstream and downstream sides of the local node,respectively. Note that the port ID has the same granularity as that ofphysical link, and hence it is equivalent to the granularity ofwavelength.

[0063] Since the nodes of the present invention do not recognize packetor cell boundaries, and hence their forwarding decision is based ontimeslots, the upstream and downstream label fields 26 and 29 of eachpath entry are used to indicate the timeslot ID on the upstream anddownstream sides of the local node, respectively. If a plurality oftimeslots exist in a path such as STM-4, the first timeslot ID of thefour timeslots of STM-4 is indicated.

[0064] Path recovery type field 30 of each path table entry is used toindicate the type of fault recovery of the path. The path recovery typeof each of the transport paths 10 and 11 is either DEDICATED 1+1,DEDICATED 1:1 or SHARED. Since the traffic path 12 is in itself notrecovered, the path recovery type of this path 12 is indicated asUNPROTECTED.

[0065] The activity field 31 of each path table entry is used toindicate whether the path is active or inactive. The accommodated pathfield 32 of each entry is used to indicate the identity of a path whichis accommodated in the transport path of the entry. The identity of thetraffic path 12 will be indicated in both entries of transport paths 10and 11.

[0066] Although not shown in FIG. 3A, each entry of the path tableincludes an additional field for indicating whether the path is aworking or a protection path, regardless of whether the path is “active”or “inactive”. In the illustrated embodiment, the transport paths 10 and11 are working and protection paths, respectively.

[0067] Switch table 8, shown in FIG. 3B, has a plurality of entries 40each having a plurality of fields 41 to 45 for setting an input port ID,an input timeslot ID, an output port ID, an output timeslot ID and thebandwidth. Switching data to be stored in the switch table 8 of eachnode will be derived from the routing data of path table 7 by having thecontroller 5 execute the routine illustrated in the flowcharts of FIGS.4A to 4C.

[0068] Examples of the path table 7 of nodes B, M, N and C areillustrated in FIGS. 5A, 6A, 7A and 8A, respectively, and examples ofthe switch table 8 of these nodes are shown in FIGS. 5B, 6B, 7B and 8B,respectively.

[0069] The following is a description of operation of the controller 5according to the flowcharts of FIGS. 4A to 4C for creating switchingdata from the routing data of path table 7 and storing the switchingdata into the switch table 8.

[0070] At step 101, the controller 5 reads data from an entry specifiedby an address pointer “i” (where i is initially equal to 1) of pathtable 7 and determines, at step 102, whether its fault recovery andactivity fields 29 and 30 indicates that (DEDICATED 1:1 and INACTIVE) or(SHARED and INACTIVE). If this is the case, flow proceeds to step 140 tocheck to see if all entries have been read. If not, flow proceeds tostep 141 to increment the address pointer “i” by one, reads data fromthe next path table entry and returns to step 102.

[0071] If the decision at step 102 is negative, flow proceeds to step103 to check to see if values are set in both UPSTREAM and DOWNSTREAMI/F ID fields 24, 27. If no values are set in both of these fields,steps 140 and 141 will be repeated to move the address pointer to thenext path entry. Therefore, the first and second entries of the node-Bpath table 7 will be ignored and the address pointer will move to thethird path entry (FIG. 5A).

[0072] If values are set in both UPSTREAM and DOWNSTREAM I/F ID fields24 and 27, flow proceeds from step 103 to step 104 to determine whetherthe UPSTREAM I/F ID is equal to a PORT ID or VL I/F ID. If the UPSTREAMI/F ID is equal to a PORT ID, flow proceeds from step 104 to step 105 tocopy the UPSTREAM I/F ID of current entry “i” to the INPUT PORT ID field41 of one entry of switch table if the path recovery type is dedicated1:1 or SHARED and additionally copy the same ID to the same field 41 ofa second entry of the switch table if the path recovery type isdedicated 1+1. At step 106, the UPSTREAM LABEL of current entry “i” iscopied to the INPUT TIMESLOT ID field 42 of one entry of switch table ifthe path recovery type of this path is dedicated 1:1 or SHARED andadditionally copied to the same field 42 of a second entry of the switchtable if the path recovery type is dedicated 1+1.

[0073] Therefore, as shown in FIGS. 5A and 5B, when the third path entry(i.e., traffic path 12) of the path table 7 of LSP-initiation node B isprocessed, the upstream interface ID=B2 is copied to the INPUT PORT IDfield 41 of two entries of the node-B switch table 8. In the transitnodes M and N, the PORT IDs M3 and N2 are copied to the INPUT PORT IDfields 41 of one entry of their switch tables and in the same entry theUPSTREAM LABEL=1 is copied from the path table to INPUT TIMESLOT IDfield 42 of the switch table (see FIGS. 6A, 6B, 7A, 7B).

[0074] If the decision at step 104 indicates that the UPSTREAM I/F ID ofthe current entry is equal to VL I/F ID, flow proceeds to step 107 tosearch the VL I/F ID field 22 for detecting path entries having the sameVL I/F ID as one contained in the UPSTREAM I/F ID field 25 anddetermines the number of detected transport paths (step 108).

[0075] This situation occurs in node C. As shown in FIG. 8A, the pathtable of node C indicates that its UPSTREAM I/F ID 25 contains C-VL1.Therefore, flow proceeds from step 104 to step 107 in which thecontroller 5 detects two path entries in the VL I/F ID field 22 havingthe same C-VL1 as one contained in the upstream interface ID field 25and determines that there are two transport paths 10 and 11 (step 108).

[0076] If only one transport path is detected, flow proceeds from step108 to step 109 to copy the UPSTREAM I/F ID of the detected path entryto the INPUT PORT ID field 41 of the switch table. At step 110, thecontroller sets the INPUT TIMESLOT ID field 42 with a value equal to(UPSTREAM LABEL of the detected entry plus UPSTREAM LABEL of currententry) minus 1.

[0077] If the number of the detected paths (i.e., path entries) is two,flow proceeds from step 108 to step 111 (see FIG. 4B) to select one ofthe detected entries which is set to ACTIVE state and copy the UPSTREAMI/F ID of the selected entry to the INPUT PORT ID field 41 of switchtable (step 112). The INPUT TIMESLOT ID filed 42 of switch table is setwith a value equal to (the UPSTREAM LABEL of the selected path entryplus UPSTREAM LABEL of current entry) minus 1 (step 113).

[0078] Therefore, in the LSP-termination node C, the UPSTREAM I/F ID=C2of the detected path entry (i.e., path 10) is copied to the INPUT PORTID field 41 of the switch table, and a LABEL=1 is obtained by thecalculation and set into the INPUT TIMESLOT ID field 42, as shown inFIGS. 8A and 8B.

[0079] Following the execution of step 106, 110 or 114, the controllerproceeds to step 115 to determine if the DOWNSTREAM I/F ID is equal toPORT ID or VL I/F ID. If DOWNSTREAM I/F ID is equal to PORT ID, theDOWNSTREAM I/F ID of the current path entry is copied to the OUTPUT PORTID field 43 (step 116) and the DOWNSTREAM LABEL of the current pathentry is copied to the OUTPUT TIMESLOT ID field 44 (step 117).

[0080] In the transit nodes M and N, the PORT IDs=M7 and =N6 are copiedto the OUTPUT PORT ID fields 43 of their switch tables and a LABEL=1 isobtained by the calculation and set into their OUTPUT TIMESLOT ID field44 (see FIGS. 6A, 6B, 7A, 7B). In the same way, the LSP-termination nodeC, the PORT ID=C6 is copied from the DOWNSTREAM I/F ID field 28 to theOUTPUT PORT ID fields 43 and a LABEL=1 is obtained by the calculationand set into the OUTPUT TIMESLOT ID field 44 (see FIGS. 8A, 8B).

[0081] If the DOWNSTREAM I/F ID is equal to VL I/F ID (step 115), the VLI/F field 22 is searched (at step 118) for detecting path entries havingthe same VL I/F ID as the one contained in the DOWNSTREAM I/F ID field28. If only one path entry is detected (step 119), the DOWNSTREAM I/F IDof the detected path entry is copied to the OUTPUT PORT ID field 43(step 120) and the OUTPUT TIMESLOT ID field 44 is set with a value equalto (DOWNSTREAM LABEL of the detected path entry plus DOWNSTREAM LABEL ofthe current path entry) minus 1 (step 121).

[0082] If two path entries are detected at step 119, flow proceeds tostep 122 (FIG. 4C) to determine whether the path recovery type of thepaths is DEDICATED 1+1. If this is the case, one of the path entriesdetected at step 118 is selected (step 123) and the DOWNSTREAM I/F ID ofthe selected path entry is copied to the OUTPUT PORT ID field 43 of afirst entry of switch table 8 (step 124). At step 125, the OUTPUTTIMESLOT ID field 44 of the first switch-table entry is set with a valueequal to (DOWNSTREAM LABEL of the selected path entry plus DOWNSTREAMLABEL of the current path entry) minus 1. In a similar manner, the otherof the detected path entries is then selected (step 126) and theDOWNSTREAM I/F ID of the selected path entry is copied to the OUTPUTPORT ID field 43 of a second entry of switch table 8 (step 127). At step128, the OUTPUT TIMESLOT ID field 44 of the second switch-table entry isset with a value equal to (DOWNSTREAM LABEL of the selected path entryplus DOWNSTREAM LABEL of the current path entry) minus 1.

[0083] Therefore, in the LSP-initiation node B, DOWNSTREAM I/F IDs=B6and =B7 are respectively copied to the OUTPUT PORT ID fields 43 of thefirst and second entries of switch table 8 and a LABEL=1 is obtained bythe calculation and set into the OUTPUT TIMESLOT ID field 44 of bothswitch table entries, as shown in FIGS. 5A and 5B.

[0084] If the decision at step 122 is negative, flow proceeds to step129 to select one of the detected path entries which is set ACTIVE. TheDOWNSTREAM I/F ID of the selected path entry is copied to the OUTPUTPORT ID field 43 (step 130), and the OUTPUT TIMESLOT ID field 44 is setwith a value equal to (DOWNSTREAM LABEL of the selected path entry plusDOWNSTREAM LABEL of the current path entry) minus 1 (step 131).

[0085] Upon completion of step 117, 121, 128 or 131, flow returns tostep 140 (FIG. 4A) to repeat the process until all path entries areprocessed.

[0086] In this way, the switch tables of all network nodes are set withswitching data. All controllers 5 of the network operate according tothe switching data stored in their respective switch tables 8 forestablishing a connection in the associated cross-connect switches 2.During network operation, the termination node of a transport pathconstantly monitors the path for detecting a failure. If the terminationnode receives a path AIS (alarm indication signal) signal, there is afailure on the path.

[0087] In FIG. 9, when an AIS signal is received on the (step 201), thecontroller proceeds to decision step 202. If the local node is not thetermination node of the faulty path, the controller ignores the messageand terminates the routine. If the local node is the termination node ofthe faulty path, the controller 5 proceeds to step 203 to examine thepath recovery type field of the faulty-path table entry and determineswhich recovery type is indicated. If the path recovery type is DEDICATED1+1, flow proceeds to step 204 and if the path recovery type isDEDICATED 1:1 or SHARED, flow proceeds to step 211 for sending asignaling message to the network before proceeding to step 204.

[0088] At step 204, the content of the ACTIVITY field of the faulty-pathentry is changed from ACTIVE to INACTIVE, and the VL I/F ID field issearched for a path entry having the same VL I/F ID as the one containedin the path entry of the faulty path (step 205). If one is found (step206), the content of the ACTIVITY field of the detected path entry ischanged from INACTIVE to ACTIVE (step 207). Therefore, if thetermination node C has detected a failure in the working transport path10, the controller 5 will update the ACTIVITY field of its path table asshown in FIG. 10A.

[0089] With the path table being updated, the controller 5 then executesthe flowcharts of FIGS. 4A, 4B and 4C to update the switch table (step208), so that the input port ID changes from C2 to C3 as shown in FIG.10B, and operates the cross-connect switch 2 according to the updatedswitch table (step 209).

[0090] As a result, the cross-connect switch 2 of node C is reconfiguredfor switching its traffic path to node D from the working path 10 to theprotection path 11. FIG. 10B indicates that an incoming STM-1 signalthat begins with the first timeslot now appears at the input port C3 andan outgoing STM-1 signal beginning with the first timeslot is deliveredto the node D from the output port C6.

[0091] If the decision at step 206 is negative, an alarm is generated(step 210).

[0092] In the foregoing description it is assumed that the informationcarried on the traffic path 12 is an STM-1 signal and each of thetransport paths 10, 11 is an STM-4 signal. Therefore, additional threeSTM-1 signals can be further accommodated in the virtual link betweennodes B and C.

[0093] Because of the hierarchical structure, the virtual link betweennodes B and C sits on a different layer from the layer on which thetraffic signal is transmitted. Thus, the bandwidth resource of thevirtual link can be arbitrarily determined independently of thebandwidth of the traffic. Hence, the virtual link can be recovered withan arbitrarily chosen granularity. If finer granularity is desired, anarrower bandwidth may be assigned to the virtual link.

[0094] Another advantage of the present invention is that the networkcan be recovered from a node failure which can occur in any transit nodebetween the initiation and termination nodes of a virtual link.

[0095] A further advantage of the present invention is that thetermination node of a virtual link is not required to perform faultlocation which is a time-consuming task. The present invention hasreduced the task of a termination node to simple functions of detectingthe presence of a failure in the virtual link and of performing aswitchover operation.

[0096] A still further advantage of the present invention is that sincea virtual link (i.e., a pair of working and protection transport paths)can be established in any network section where protection is desired,the fault recovery method that must be implemented is only required tosupport the end-to-end path protection scheme of the initiation andtermination nodes. It is not necessary to support other fault recoverymethods. Since the two transport paths have a definite initiation pointwhere they diverge and a definite termination point where they converge,the node that performs path splitting, the node that performs pathmonitoring and the node that performs path switching can beautomatically determined and easily identified. Accordingly, the pathsetup and path switchover operation of these nodes are simplified.

[0097] A still further advantage of the present invention is that when afailure occurs in the virtual link it is not necessary at all to updatethe traffic-path entry of the path table. In other words, the controlfunction of the traffic path can be completely hidden behind the faultrecovery operation of the transport path.

[0098] Additionally, the physical links that comprise the transportpaths are “unprotected” at the link layer. The fault recovery operationon the transport paths would not encounter contention which wouldotherwise occur with the link layer.

[0099]FIG. 11 is a block diagram of a distributed network in which therouting functions of the network are distributed among theinterconnected nodes. In this embodiment, the nodes B, M, N and C areconnected to form a transport network and the nodes A and D are assumedto be the clients of the transport network. Client nodes A and D areconnected to the transport network via a user network interface (UNI)and the nodes B, M, N, C are interconnected by a network node interface(NNI).

[0100] Line interfaces 6 of adjacent nodes are interconnected viabi-directional control channels. Specifically, the control channel forinterconnecting adjacent nodes A and B and the control channel forinterconnecting adjacent nodes C and D are UNI control channels forexchanging signaling messages according to a UNI signaling protocol.Request messages (packets) such as path setup or path release are sentfrom the client nodes A and D to the transport nodes B and C,respectively.

[0101] The control channels used to interconnect the adjacent nodes ofthe transport network are NNI signaling channels on which routingmessages are transported according to a routing protocol. A topologydatabase is maintained in each of the transport nodes B, M, N and C sothat they know the initiation and termination points of all links, theirmaximum bandwidths, the available bandwidths, and their link-faultrecovery types.

[0102] Although the network of FIG. 11 can operate in dedicated 1+1fault recovery mode, the following is a description of the networkoperating in a dedicated 1:1 recovery mode.

[0103] Client node A initially sends a connection request message overthe control channel 9 to the node B to request for the establishment ofan STM-1 path, i.e., the traffic path 12, to the client node D,containing the path ID (i.e., “12”), the bandwidth resource (i.e.,STM-1), the identifiers of the initiation and termination nodes of thepath (i.e., A and D), and the link-fault recovery type (i.e., dedicated1:1).

[0104] In response to this connection request message, the node Bdetermines that transport paths 10 and 11 be set up between nodes B andC and makes the determination of bandwidth (i.e., STM-4) for each ofthese transport paths. By using its own topology database, the node Bprovides a route calculation for the paths 10 and 11 so that they form anode-disjoint relationship with each other and that they pass throughlinks whose link-fault recovery type is “unprotected”. Therefore, thelinks comprising the transport paths are not restored at the link layerwhen a link fault occurs in any of these links. If the node B hasdetermined that a route B-M-C is the working transport path 10 and aroute B-N-C is the protection transport path 11, it sends a labelrequest message to node M, containing the path ID (i.e., “10”), a tunnelID (=B−1, for example), the bandwidth resource (i.e., STM-4), androuting information indicating the identifiers of nodes B, M, C in whichthe traffic path is to be accommodated, a path recovery type (i.e.,dedicated 1:1) and link recovery type (i.e., unprotected), aworking/protection indication. Note that the tunnel ID is used toindicate which transport paths form a common virtual link.

[0105] Upon receipt of the label request message from node B, thecontroller 5 of node M creates its own path table 7 by setting it withthe information contained in the received message, and then retransmitsa copy of the message to node C.

[0106] Node C responds to this message by creating its own path table inthe same manner as node M has performed on the received message. Sincethe node C knows that it is the termination point of the workingtransport path 10 from the routing information contained in the message,the label request message is no longer transmitted downstream. Node Cassigns VL I/F ID=C-VL1 to the working transport path 10 and sets thisID into its path table (see FIG. 15A). Node C makes a search through itstopology database and selects an unprotected link (C2, for example)having a sufficient available bandwidth for a STM-4 path and determinesa timeslot (i.e., ID=1, for example) as a starting slot of fourcontinuous STM-1 signals. Node C sets the link's interface identifier C2into the UPSTREAM I/F ID field and the timeslot ID into the UPSTREAMLABEL field of the path table (FIG. 15A). Subsequently, the node Ctransmits a label assignment message to node M, containing the assignedUPSTREAM I/F ID (=C2) and the UPSTREAM LABEL (=1) as well as theinformation contained in the received label request message.

[0107] On receiving the label assignment message from node C, the node Mcreates its own path table by writing the UPSTREAM I/F ID and UPSTREAMLABEL contained in the message into the DOWNSTREAM I/F ID and DOWNSTREAMLABEL fields of the path table, respectively. Since the node M knowsthat port M7 is connected to port C2 (see FIG. 2), port ID=M7 is set inthe DOWNSTREAM I/F ID field of its path table (FIG. 13A). Node Mdetermines the link and timeslot numbers to be assigned to this path inthe same manner as node C has determined its link and timeslot andwrites the determined identifiers into the UPSTREAM I/F ID and UPSTREAMLABEL fields of its path table, respectively, and then rewrites thecorresponding identifiers of the received label assignment message withthe determined identifiers and sends it to node B.

[0108] Node B performs a similar process on the label assignmentmessaged received from the node M, but does not transmit the messagefurther to the client node A as the message indicates that node B is theinitiation point of the path 10. Node B assigns B-VL1 to the path 10 andsets it into the VL I/F field of its own path table (FIG. 12A).

[0109] With the working transport path 10 being established, the node Bproceeds to establish the protection transport path 11 by sending alabel request message to node N, using the same tunnel ID (=B−1) as thatused in the path 10. Data associated with the protection path 11 are setinto the second entry of the path table of nodes B and C as indicated inFIGS. 12A and 15A, and the node N creates its own path table as shown inFIG. 14A.

[0110] When the client node A sends a signaling message to node B, forrequesting the establishment of a connection to the node D, the pathtables of initiation and termination nodes B and C are updated accordingto the data contained in the connection request message.

[0111] Nodes B, M, N and C proceed to create their switch table 8 byperforming the routine as described with reference to the flowcharts ofFIGS. 4A, 4B and 4C. As a result, switch tables are created in the nodesB, M and C as indicated in FIGS. 12B, 13B and 15B, but no switch tableis created in the node N (FIG. 14B). Since the path recovery type of thetransport path 10 is assumed to be dedicated 1:1, the data associatedwith the protection path 11 are not set into the switch table.Therefore, switching data is set into only one entry of each of theseswitch tables.

[0112] If the node C detects a failure in the working path 10, itchanges the ACTIVITY field of its path table to “INACTIVE” by executingthe flowchart of FIG. 9 and changes the INPUT PORT ID field of itsswitch table to C3 according to the flowcharts of FIGS. 4A-4C, asindicated in FIG. 16A. The node C transmits a signaling message to nodeN. In response, the node N changes the ACTIVITY field of its path tableto “ACTIVE”, creates its own switch table according to FIGS. 4A-4C, asshown in FIG. 16B, and transmits the signaling message to node B.Similar to the node C, the node B updates the ACTIVITY field of its pathtable and changes the OUTPUT PORT ID field of its switch table to B7 asindicated in FIG. 16C.

[0113] Transport paths can be concatenated as shown in FIG. 17A. In thisconfiguration, nodes A and F are client nodes interconnected by atransport network comprising nodes B, M, N, C, O, P and D,interconnected by optical links. The transport network is divided into afirst section B-C and a second section C-D so that nodes B and C arerespectively the initiation and termination nodes of transport paths 13,14, and the nodes C and D are respectively the initiation andtermination nodes of transport paths 15, 16. The termination node ofeach network section monitors the working transport path.

[0114] If the path recovery type of the network is dedicated 1+1, thesignal on the traffic path 12 from client node A is split into two inthe node B and respectively transmitted over the transport paths 13 and14 to the node C, where the signal from the path 13 (i.e., the inputport C1) is split and respectively sent from the output ports C3 and C4over the transport paths 15 and 16 to the node D. Node D transmits thesignal on path 15 to the client node E. When a failure occurs in thepath 13 as indicated in FIG. 17B, the node C clears the connectionbetween ports C1 and C4 and reestablishes connections so that the signalon path 14 (i.e., the input port C2) is supplied to the output ports C3and C4.

[0115] In this network configuration, the granularity of fault recoverycan be independently determined for network sections B-C and C-D. Forexample, the section B-C is implemented with two STM-4 paths, while thesection C-D is implemented with two paths of a different SDH layer iffiner granularity is desired for section C-D. Another advantage of thisinvention is that, compared to a network where the nodes B and D wereinterconnected by a single span of transport paths, the time taken torecover from a failure can be reduced since the fault recovery time isproportional to the length of each network section through whichsignaling messages are transmitted for restoration. This is particularlyadvantages if the fault recovery type is N:M protection mode.

[0116] Fault recovery contention between an SDH layer and a wavelengthlayer can be avoided in a communications network of FIG. 18.

[0117] In this network, client nodes A and H are interconnected by atransport network comprising nodes B, M, N, C, D, O, P, E, F, Q, R andG. The network is segmented into a plurality of domains 50, 51 and 52,so that nodes B, M, N, C comprise the domain 50, nodes D, O, P, Ecomprise the domain 51, and nodes F, Q, R, G comprise the domain 52.

[0118] In each of these domains, adjacent nodes are interconnected byoptical links. Nodes M, O and Q are interconnected by inter-domaindirect links 60 and 61 and nodes N, P and R are interconnected byinter-domain direct links 62 and 63. Nodes C and D are interconnected byan optical link 64 and nodes E and F are interconnected by an opticallink 65. In the domain 51, all adjacent nodes are interconnected byoptical links having wavelengths W1, W2, W3, W4 as illustrated.

[0119] Initially, the node B performs a route calculation for routes tonode C and sends a signaling message to node C to establish a workingtransport path 13 via node M and a protection transport path 14 via nodeN, using respective links whose link recovery type is “unprotected”.Using a routing protocol, the nodes B and C advertise these paths as avirtual link having virtual link (VL) interface identifiers (B-VL1) and(C-VL1), respectively, and their link recovery type as “dedicated 1+1”.

[0120] Node D performs route calculation for working and protectiontransport paths 15 and 16 in a similar manner to that of node C. Then,the node D sends a label request message to establish the calculatedpaths 15 and 16. Since the paths 15 and 16 are wavelength paths, opticalfiber IDs (=W1 and W2) are indicated in the respective entries of theDOWNSTREAM I/F ID field of node-D path table (FIG. 19) and portidentifiers (=D2 and D3) are indicated in the respective path entries ofthe DOWNSTREAM LABEL field.

[0121] For the path 15, for example, the node E performs a similarprocess and creates its own path table in which it assigns optical fiberID=W3 and places this optical fiber ID in its path table entry of theUPSTREAM I/F ID field, and places port ID (=E2) in its path table entryof the UPSTREAM LABEL field and sends a label assignment messageupstream and a label request message downstream.

[0122] Node O responds to this label assignment message by writing W3into the DOWNSTREAM I/F ID field of its own path table and output portID (=O2) into the DOWNSTREAM LABEL field, and sends a label assignmentmessage upstream, indicating W1 and O1 in the UPSTREAM I/F ID andUPSTREAM LABEL fields of the message, respectively.

[0123] Node D responds to the label assignment message from node 0 bywriting W1 and D2 into the DOWNSTREAM I/F ID and DOWNSTREAM LABEL fieldsof its own path table.

[0124] Transport paths 15 and 16 form a single virtual link, which isadvertised to all network nodes as having an interface identified asD-VL1 as viewed from node D and an interface identified as E-VL1 asviewed from node E. Because of the contention-free characteristic of thefault recovery scheme of the present invention, the link recovery typeof the transport paths 15, 16 is “unprotected” at the link layer andtheir path recovery type is “dedicated 1+1” at the SDH layer.

[0125] In the same way, the node F performs a route calculation andestablishes transport paths 17 and 18 to node G, which are advertised toall network nodes as having VL interface identifiers F-VL3 and G-VL3 asviewed from nodes F and G, respectively.

[0126] In response to the advertisement message from node F, the node Cperforms a route calculation for a transport path (SDH path) 19 to thenode F, exclusively using links whose link recovery type is “dedicated1+1” and finds a route C-D-O-E-F via the wavelength path 15 and sends asignaling message along this route for setting up the path. As indicatedin FIG. 19, the path recovery type of the SDH path 19 is specified as“unprotected”. Then the node C advertises the SDH path 19 as a virtuallink which has VL interface identifiers C-VL4 and F-VL4 as viewed fromnodes C and F, respectively.

[0127] Consider a route that is comprised of a series of concatenatedlinks. If one of the links is an “unprotected” link and the other linksare of “1+1”, “1:1” or “shared” protection type, the unprotected link isthe most unreliable link of the route. According to the presentinvention, the recovery type of a virtual link, which comprises a seriesof component links, is set equal to the most unreliable recovery type ofits component links, and the virtual link is advertised as having arecovery type equal to the most unreliable recovery type of itscomponent links. Since the link recovery types of all the componentlinks 64, 15 (=W1+W3) and 65 of virtual link 19 are the same “dedicated1+1”, the link recovery type of the transport path (virtual link) 19 isadvertised as “dedicated 1+1”. If the recovery type of one of thesecomponent links is dedicated 1:1, for example, which is less reliablethan dedicated 1+1, the recovery type of virtual link 19 will beadvertised as dedicated 1:1.

[0128] The transport network is now ready to accept requests from theclient nodes. When the client node A sends a signaling message to node Bfor setting up the traffic path 12 to the client node H, the node B useslinks of the dedicated 1+1 recovery type to node G to perform a routecalculation. The result of this calculation would result in theestablishment of a route A-B-C-F-G-H by exchanging signaling messagesbetween nodes B and G, so that the path 12 has its sections B-C, C-F andF-G accommodated respectively in the protected VL-1 formed by paths 13and 14, the unprotected VL formed by path 19, and the protected VL-2formed by paths 17 and 18.

[0129] When the traffic path 12 has been set up in the network, thenodes C, E and G monitor the working paths 13, 15 and 17, respectively.Since the path 19 is of unprotected path recovery type, the terminationnode F of this path performs no monitoring on this path. If thewavelength path should fail, fault recovery is performed at thewavelength layer. Since the optical links W1 to W4 are unprotected, nofault recovery contention occurs with the SDH layer. Since the links 64and 65 are of “dedicated 1+1” type, while the path 19 is unprotected, nofault recovery contention occurs if any of these links should fail.

[0130] The provision of the path 19 is advantageous if it is desired toexplicitly perform the monitoring of a section of a traffic path such asthe section C-F or desired to make such a section invisible to theclient node A.

[0131] If the node B fails to discover a high-speed fault recovery routeto the node G using only dedicated 1+1 links as illustrated in FIG. 18,the node B then performs a route calculation for a low-speed recoverypath using only “unprotected” links for “disjoint” paths 70 and 71 tothe node G, as shown in FIG. 20. If a route B-M-O-Q-G connected by links60 and 61 is found for the path 70 and a route B-N-P-R-G connected bylinks 62 and 63 is found for the path 71, the node B sends signalingmessages over these routes to indicate that the path recovery type ofthe paths 70 and 71 is “dedicated 1+1”. Node B sets the paths 70 and 71as a “dedicated 1+1” virtual link in its own topology database. Node Bthen performs a calculation for a “dedicated 1+1” route to the node Gand finds the paths 70 and 71. Node B sends signaling messages over thedetected paths to establish the traffic path 12.

[0132] In a network where inter-domain direct links such as links 60,61, 62, 63 are not provided, the link recovery type of inter-domainlinks 64 and 65 may be set in a “DON'T CARE” mode. With this recoverymode setting, these links can be used as candidates in routecalculations of any fault recovery mode.

[0133] According to another aspect of the present invention, amulti-domain network is illustrated in FIG. 21, in which each domainoperates according to the domain-specific routing protocol. Each node ofthis network has a topology database of its own domain and knows aboutthe edge nodes to which the client nodes of the domain are connected,and about the transit nodes of its own domain through which a connectionis set up to a destination node outside of its own domain. Border nodesof each domain know about their own domain identifier and theidentifiers of transit nodes through which a connection is set up to adestination node of another domain. In such a multi-domain network, thepresent invention enables all domains of the network to consistentlyoperate in a 1+1 protection mode on the traffic path 12.

[0134] In a multi-domain network as shown in FIG. 21, the client node Ainitially sends a signaling message (i.e., user-to-network-interfacesignaling) to the node B for requesting the establishment of trafficpath 12 to the client node H over a “1+1 protection” route. Since thenode B knows that the route must pass through the node C in order toreach the destination, it attempts to find a route to the node C onwhich “1+1 protection” is performed. Since such a route is notavailable, the node B performs a route calculation for a pair ofmutually “disjoint” paths 13 and 14 to the node C and establishes thesepaths by sending a signaling message (using internal network-to-networkinterface signaling) to the node C. After setting the paths 13 and 14into its own topology database as a virtual link, the node Brecalculates a route for the traffic path 12, which would result in thedetection of a route that passes through this VL to the node C. Then,the node B transmits a label request message to the node C, containing apath ID (=B−1), the initiation point of path 12 (=A), the terminationpoint of path 12 (=H), routing information (B-C) to the node C, thebandwidth required, and the link recovery type (=dedicated 1+1).

[0135] Since the node C knows that for a route to reach the destinationnode H such a route must pass through domain 51, it responds to thelabel request message from the node B by sending a label request message(using external network-to-network interface signaling) to the node D,containing the same information as that of the received message exceptfor the routing data.

[0136] Since the node D knows that for a route to reach the destinationnode H the route must pass through the node E, it performs routecalculation for paths 15 and 16 and establishes these paths as a virtuallink and sends a label request message to the node E for requesting thetraffic path 12 in the same manner as the node B has performed forestablishing the paths 13 and 14.

[0137] Since the node E knows that for a route to reach the destinationnode H the route must pass through domain 52, it responds to the labelrequest message from the node D by sending a label request message tothe node F.

[0138] Knowing that the destination node H is connected to the node G,the node F performs route calculation for paths 17 and 18 andestablishes these paths as a virtual link and sends a label requestmessage to the node G for requesting the traffic path 12 in the same waythe path 12 has been established over the paths 13, 14 and 15, 16. NodeG, on receiving the label request message from node F, retransmits it tothe destination node H.

[0139] In response, the node H assigns an UPSTREAM I/F ID and anUPSTREAM LABEL to the traffic path 12 and sends a label assignmentmessage to the node G, containing the assigned information.

[0140] Node G translates the informed label and ID to DOWNSTREAM I/F IDand DOWNSTREAM LABEL and stores the translated ID and label informationinto the path-table entry of traffic path 12. Node G assigns an UPSTREAMI/F ID and an UPSTREAM LABEL to the traffic path 12 according to theinformation contained in the label request message it has received fromthe node F, and sends a label assignment message to the node F,containing the assigned information.

[0141] On receiving the label assignment message from node G, the node Ftranslates the upstream information contained in it to DOWNSTREAM I/F IDand DOWNSTREAM LABEL and stores them into its own path table and assignsan UPSTREAM I/F ID and an UPSTREAM LABEL to the traffic path 12according to the information contained in the label request message ithas received from the node E, and sends a label assignment message tothe node E, containing the assigned information.

[0142] A similar process is successively repeated in the domains 51 and50, so that in the nodes E, D, C and B interface IDs and labels arerespectively assigned to the traffic path 12. Finally, the node B sendsa label assignment message to the client node A, containing the UPSTREAMI/F ID and UPSTREAM LABEL it has assigned to the traffic path 12.

[0143] Client node A translates the received ID and label to DOWNSTREAMI/F ID and DOWNSTREAM LABEL and stores them into the path-table entry ofthe traffic path 12.

[0144] Monitoring operation now begins at nodes C, E and G respectivelyon the working paths 13, 15 and 17. In this way, 1+1 protection of thetraffic path 12 can be implemented in the domains 50, 51 and 52.

What is claimed is:
 1. A communications network comprising: first andsecond nodes interconnected by communication links; and a traffic pathestablished between a source node and a destination node; each of saidfirst and second nodes including routing control means for establishingfirst and second transport paths on a first fault recovery layer betweensaid first and second nodes, said first node further including adatabase for storing identities of said first and second transport pathsas a single virtual link as viewed from said source node, said firstnode advertising fault recovery type of said virtual link as being equalto fault recovery type of said first and second transport paths, saidfirst and second nodes accommodating said traffic path on a second faultrecovery layer through said first transport path, and accommodating saidtraffic path on said second fault recovery layer through said secondtransport path when said first transport path is not working properly.2. A communications network comprising: first and second nodesinterconnected by communication links; and a traffic path establishedbetween a source node and a destination node; each of said first andsecond nodes including routing control means for establishing first andsecond transport paths on a first fault recovery layer between saidfirst and second nodes, said first node further including a database forstoring identities of said first and second transport paths as a singlevirtual link as viewed from said source node, said virtual linkcomprising a series of component links, said first node advertisingfault recovery type of said virtual link as being equal to mostunreliable fault recovery type of said component links, said first andsecond nodes accommodating said traffic path on a second fault recoverylayer through said first transport path, and accommodating said trafficpath on said second fault recovery layer through said second transportpath when said first transport path is not working properly.
 3. Thecommunications network of claim 2, wherein said first node advertisesfault recovery type of said virtual link as being equal to faultrecovery type of said first and second transport paths.
 4. Thecommunications network of claim 1 or 2, wherein said first and secondfault recovery layers are hierarchically structured in such arelationship that no contention occurs between the first and secondfault recovery layers when a failure occurs in said first and secondtransport paths.
 5. The communications network of claim 1 or 2, whereinsaid first node is an initiation node of said traffic path and saidsecond node is a termination node of said traffic path.
 6. Thecommunications network of claim 1 or 2, wherein said first and secondtransport paths do not share a common physical link.
 7. Thecommunications network of claim 6, wherein said first and secondtransport paths do not pass through a common node other than said firstand second nodes.
 8. The communications network of claim 1 or 2, whereinsaid second node is downstream of said first transport path and monitorssaid first transport path for detecting a failure.
 9. The communicationsnetwork of claim 1 or 2, wherein said first node supplies a signal onsaid traffic path to said first and second transport pathssimultaneously and wherein said second node receives said signal fromsaid first transport path when the first transport path is workingproperly and receives said signal from said second transport path whenthe first transport path is not working properly.
 10. The communicationsnetwork of claim 1 or 2, wherein said first node supplies a signal onsaid traffic path to said first transport path and said second nodereceives said signal from said first path when the first transport pathis working properly, and wherein said first node supplies said signal tosaid second transport path and said second node receives said signalfrom said second transport path when said first transport path is notworking properly.
 11. The communications network of claim 1 or 2,wherein said first node supplies a signal on said traffic path to saidfirst transport path and said second node receives said signal from saidfirst transport path when the first transport path is working properly,and wherein said first node supplies said signal to said first andsecond transport paths simultaneously and said second node receives saidsignal from said second transport path when said first transport path isnot working properly.
 12. The communications network of claim 1 or 2,wherein each of said first and second transport paths is established onan unprotected transmission medium.
 13. The communications network ofclaim 1 or 2, wherein each of said first and second nodes comprises: afirst table memory for storing routing data of said first and secondtransport paths and said traffic path; a second table memory for storingswitching data derived from said routing data; a switch for terminatingsaid first and second transport paths; and a controller for establishinga connection in said switch according to said switching data so thatsaid traffic path is accommodated in said first transport path, saidcontroller responding to an occurrence of a failure in said firsttransport path for updating said routing data, updating the switchingdata according to the updated routing data and reestablishing aconnection in said switch according to the updated switching data sothat said traffic path is accommodated in said second transport path.14. The communications network of claim 13, wherein said controllerderives said switching data from said routing data by translation from acontrol plane of said network to a transport plane of said network. 15.The communications network of claim 14, wherein said control plane is aGMPLS (generalized multi-protocol label switching) system and saidtransport plane is an SDH/SONET system.
 16. The communications networkof claim 14, wherein said controller updates said routing data when saidfailure is detected in said first transport path, updates said switchingdata according to the updated routing data and reestablishes saidconnection according to the updated switching data.
 17. Thecommunications network of claim 1 or 2, wherein each of said first andsecond transport paths has a bandwidth resource greater than a bandwidthresource of said traffic path.
 18. The communications network of claim 1or 2, wherein each of said first and second transport paths is awavelength path.
 19. A multi-domain communications network comprising: aplurality of network domains interconnected by inter-domain links, eachof said network domains including first and second nodes interconnectedby inter-node links; and a traffic path established between a sourcenode and a destination node, the first node, of each of said domainsestablishing first and second parallel transport paths on a first faultrecovery layer to the second node of the domain and including a databasefor storing identities of said first and second parallel transport pathsas a single virtual link as viewed from said source node, andaccommodating said traffic path on a second fault recovery layer throughthe first transport path of the domain, said virtual link comprising aseries of component links, said first node advertising fault recoverytype of said virtual link as being equal to fault recovery type of saidfirst and second transport paths, said first and second nodes of each ofsaid domains accommodating said traffic path in the second transportpath of the domain when the first transport path of the domain is notworking properly.
 20. A multi-domain communications network comprising:a plurality of network domains interconnected by inter-domain links,each of said network domains including first and second nodesinterconnected by inter-node links; and a traffic path establishedbetween a source node and a destination node, the first node of each ofsaid domains establishing first and second parallel transport paths on afirst fault recovery layer to the second node of the domain andincluding a database for storing identities of said first and secondparallel transport paths as a single virtual link as viewed from saidsource node, and accommodating said traffic path on a second faultrecovery layer through the first transport path of the domain, saidvirtual link comprising a series of component links, said first nodeadvertising fault recovery type of said virtual link as being equal tomost unreliable fault recovery type of said component links, said firstand second nodes of each of said domains accommodating said traffic pathin the second transport path of the domain when the first transport pathof the domain is not working properly.
 21. The multi-domaincommunications network of claim 20, wherein said first node advertisesfault recovery type of said virtual link as being equal to faultrecovery type of said first and second transport paths.
 22. Themulti-domain communications network of claim 19 or 20, wherein saidfirst and second fault recovery layers are hierarchically structured insuch a relationship that no contention occurs between the first andsecond fault recovery layers when a failure occurs in said first andsecond transport paths.
 23. The multi-domain communications network ofclaim 19 or 20, wherein each of said nodes comprises: a first tablememory for storing routing data of said first, second and traffic paths;a second table memory for storing switching data derived from saidrouting data; a switch for terminating said first and second transportpaths; and a controller for establishing a connection in said switchaccording to said switching data so that said traffic path isaccommodated in said first transport path, said controller responding toan occurrence of a failure in said first transport path for updatingsaid routing data, updating the switching data according to the updatedrouting data and reestablishing a connection in said switch according tothe updated switching data so that said traffic path is accommodated insaid second transport path.
 24. The multi-domain communications networkof claim 23, wherein said controller derives said switching data fromsaid routing data by translation from a control plane of said network toa transport plane of said network.
 25. The multi-domain communicationsnetwork of claim 24, wherein said controller updates said routing datawhen said failure is detected in said first transport path, updates saidswitching data according to the updated routing data and reestablishessaid connection according to the updated switching data.
 26. Themulti-domain communications network of claim 19 or 20, wherein faultrecovery type of the inter-node links of each of said domains isunprotected, and fault recovery type of said first and second transportpaths of each of said domains is dedicated 1+1.
 27. The multi-domaincommunications network of claim 19 or 20, wherein fault recovery type ofthe inter-node links of each of said domains is unprotected, and faultrecovery type of said first and second transport paths of each of saiddomains is dedicated 1:1.
 28. The multi-domain communications network ofclaim 19 or 20, wherein fault recovery type of the inter-node links ofeach of said domains is unprotected, and said first and second transportpaths of each of said domains are of shared fault recovery type.
 29. Themulti-domain communications network of claim 19 or 20, wherein each ofsaid inter-domain links is of don't care fault recovery type.
 30. Themulti-domain communications network of claim 19 or 20, wherein saidfirst fault recovery layer is a wavelength layer and said second faultrecovery layer is a TDM (time division multiplex) layer.
 31. Themulti-domain communications network of claim 19 or 20, wherein saiddomains comprise first and second domains and an intermediate domainconnected between the first and second domains, and wherein said first,second and intermediate domains establish a third transport path on athird fault recovery layer between the second node of said first domainand the first node of said third domain through the first transport pathof said intermediate domain.
 32. The multi-domain communications networkof claim 31, wherein the first fault recovery layer of said intermediatedomain is a TDM layer and said third fault recovery layer is awavelength layer.
 33. The multi-domain communications network of claim19 or 20, wherein the first and second nodes of each of said domainstransmits a request message downstream to, and receives the requestmessage from, an adjacent node for establishing said first and secondtransport paths in each of the domains and specifying a particular faultrecovery type and a bandwidth resource of the established transportpaths and transmits a return message upstream to, and receives thereturn message from, an adjacent node, each of said nodes setting thefault recovery type specified in the received request message into afirst table memory and setting an input and output connectivity in saidfirst table memory by associating information contained in the receivedrequest message and information contained in the received returnmessage, each of said nodes creating a switch table according toinformation contained in the first table memory, each of said nodesincluding a switch for establishing a connection according to saidswitch table for transporting signals of said traffic path.
 34. Themulti-domain communications network of claim 19 or 20, wherein the firstand second nodes of each of said domains transmits a first requestmessage downstream to, and receives the first request message from, anadjacent node for establishing said first and second transport pathsusing protected inter-domain links, wherein each of said nodesdetermines whether or not a transport path specified by the receivedrequest message is discovered, and wherein, if the specified transportpath is not discovered, each of said nodes transmits a second requestmessage downstream to, and receives the second request message from, anadjacent node for establishing said first and second transport pathsusing unprotected inter-domain links.
 35. The multi-domaincommunications network of claim 19 or 20, wherein each of said first andsecond transport paths is a wavelength path.
 36. A fault recovery methodfor a communications network in which first and second nodes areinterconnected by communication links, the method comprising the stepsof: establishing first and second parallel transport paths on a firstfault recovery layer between the first and second nodes; storingidentities of said first and second parallel transport paths in adatabase as a single virtual link as viewed from a source node;advertising fault recovery type of said virtual link as being equal tofault recovery type of said first and second transport paths;establishing a traffic path between a source node and a destinationnode; accommodating said traffic path on a second fault recovery layerthrough said first transport path; and monitoring said first transportpath, at said second node, and accommodating said traffic path on saidsecond fault recovery layer through said second transport path when afailure is detected in said first path.
 37. A fault recovery method fora communications network in which first and second nodes areinterconnected by communication links, the method comprising the stepsof: establishing first and second parallel transport paths on a firstfault recovery layer between the first and second nodes; storingidentities of said first and second parallel transport paths in adatabase as a single virtual link as viewed from a source node, saidvirtual link comprising a series of component links; advertising faultrecovery type of said virtual link as being equal to most unreliablefault recovery type of said component links; establishing a traffic pathbetween a source node and a destination node; accommodating said trafficpath on a second fault recovery layer through said first transport path;and monitoring said first transport path, at said second node, andaccommodating said traffic path on said second fault recovery layerthrough said second transport path when a failure is detected in saidfirst path.
 38. The fault recovery method of claim 37, furthercomprising the step of advertising fault recovery type of said virtuallink as being equal to fault recovery type of said first and secondtransport paths.
 39. The fault recovery method of claim 36 or 37,wherein said first and second fault recovery layers are hierarchicallystructured in such a relationship that no contention occurs between thefirst and second fault recovery layers when a failure occurs in saidfirst and second transport paths.
 40. The fault recovery method of claim36 or 37, further comprising the steps of: supplying a signal of saidtraffic path from said first node to said first and second transportpaths simultaneously, and receiving the signal at said second node fromsaid first transport path when the first transport path is workingproperly; and receiving said signal at said second node from said secondtransport path when said failure is detected in said first transportpath.
 41. The fault recovery method of claim 36 or 37, furthercomprising the steps of: supplying a signal of said traffic path fromsaid first node to said first transport path and receiving said signalat said second node from said first transport path when the firsttransport path is working properly; and supplying said signal from saidfirst node to said second transport path and receiving said signal atsaid second from said second transport path when said failure isdetected in said first transport path.
 42. The fault recovery method ofclaim 36 or 37, further comprising the steps of: supplying a signal ofsaid traffic path from said first node to said first transport path andreceiving said signal from said first transport path at said second nodewhen the first transport path is working properly; and supplying saidsignal to said first and second transport paths simultaneously from saidfirst node and receiving said signal from said second transport path atsaid second node when said failure is detected in said first transportpath.
 43. The fault recovery method of claim 36 or 37, furthercomprising the steps of: a) storing routing data of said first, secondand traffic paths in a first table memory; b) storing switching dataderived from said routing data in a second table memory; c) terminatingsaid first and second transport paths to a switch; d) establishing aconnection in said switch according to said switching data so that saidtraffic path is accommodated in said first transport path; e) updatingsaid routing data when a failure occurs in said first transport path; f)updating the switching data according to the updated routing data; andg) reestablishing a connection in said switch according to the updatedswitching data so that said traffic path is accommodated in said secondtransport path.
 44. The fault recovery method of claim 43, wherein step(b) comprises deriving said switching data from said routing data bytranslation from a control plane of said network to a transport plane ofsaid network.
 45. The fault recovery method of claim 44, wherein saidcontrol plane is a GMPLS (generalized multi-protocol label switching)system and said transport plane is an SDH/SONET system.
 46. The faultrecovery method of claim 44, further comprising the steps of: updatingsaid routing data when said failure is detected in said first transportpath; updating said switching data according to the updated routingdata; and reestablishing said connection according to the updatedswitching data.
 47. The fault recovery method of claim 36 or 37, whereinsaid communications network is a multi-domain communications networkwherein a plurality of network domains are interconnected byinter-domain links, and further comprising the steps of: transmitting arequest message from each of said nodes downstream to, and receiving therequest message from, an adjacent node for establishing said first andsecond transport paths in each of the domains by specifying a particularfault recovery type and a bandwidth resource of the establishedtransport paths; transmitting a return message upstream to, and receivesthe return message from, an adjacent node; setting the fault recoverytype specified in the received request message into a first table memoryof each node and setting an input and output connectivity in said firsttable memory by associating information contained in the receivedrequest message and information contained in the received returnmessage; creating a second table memory, in each node, according toinformation contained in the first table memory of the node; andestablishing a connection in each of said nodes according to the secondtable memory of the node for transporting signals on said traffic path.48. The fault recovery method of claim 36 or 37, wherein saidcommunications network is a multi-domain communications network whereina plurality of network domains are interconnected by inter-domain links,and further comprising the steps of: transmitting a first requestmessage from each of said nodes downstream to, and receives the firstrequest message from, an adjacent node for establishing said first andsecond transport paths using protected inter-domain links; determiningwhether or not a transport path specified by the received requestmessage is discovered; and if the specified transport path is notdiscovered, transmitting a second request message from each nodedownstream to, and receiving the second request message from, anadjacent node for establishing said first and second transport pathsusing unprotected inter-domain links.